Preheat oven to 375?. Beat 1/2 cup butter at medium speed with an electric hand or stand mixer until creamy; gradually add 1/2 cup sugar, beating until light and fluffy. Add egg, and beat until blended.
Combine 1 1/4 cups flour, baking powder, and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture; beat at low speed just until blended after each addition. Stir in vanilla and lemon zest.
Pour batter into a greased 8-inch square baking dish. Top with blueberries.
Combine remaining 1/2 cup sugar, remaining 1/4 cup flour, pecans, and cinnamon. Cut in 1/4 cup butter with pastry blender or fork until mixture resembles small peas; sprinkle crumb topping over blueberries.